Practice question

**Magnetic field of bar magnet** follows inverse cube law B ∝ m/r³, unlike inverse square for electric dipole. Given B at distance r, moment m = B r³/(μ₀/4π) for equatorial, m = B r³/(2·μ₀/4π) for axial, enabling moment extraction from measured field. Diamagnetic materials exhibit weak repulsion from a magnetic field because an external field induces small currents in their atoms that generate an opposing magnetic moment, per Lenz’s law, resulting in a slight reduction of the field inside the material. Substituting values gives Induced opposing moments, which matches expected magnitude for this magnetic configuration, confirming dipole field dependence on m/r³ and torque relation τ = m B sinθ.